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Cereb Cortex. 2018 Sep 1;28(9):3159-3175. doi: 10.1093/cercor/bhx187.

Quantitative 3D Ultrastructure of Thalamocortical Synapses from the "Lemniscal" Ventral Posteromedial Nucleus in Mouse Barrel Cortex.

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Department of Anatomy & Neuroscience, School of Medicine, Autonoma de Madrid University, Madrid, Spain.
Institute of Neuroscience and Medicine INM-2, Research Centre Jülich GmbH, Jülich, Germany.
Laboratorio Cajal de Circuitos Corticales, Centro de Tecnología Biomédica, Universidad Politécnica de Madrid, Pozuelo de Alarcón, Madrid, Spain.
CIBERNED, Centro de Investigación Biomédica en Red de Enfermedades Neurodegenerativas, Madrid, Spain.
Departamento de Arquitectura y Tecnología de Sistemas Informáticos, Universidad Politécnica de Madrid, Boadilla del Monte, Madrid, Spain.
Instituto Cajal, Consejo Superior de Investigaciones Científicas, Madrid, Spain.
Department of Psychiatry, Psychotherapy and Psychosomatics, RWTH Aachen University, Aachen, Germany.
JARA-Brain Medicine, Aachen, Germany.


Thalamocortical synapses from "lemniscal" neurons of the dorsomedial portion of the rodent ventral posteromedial nucleus (VPMdm) are able to induce with remarkable efficacy, despite their relative low numbers, the firing of primary somatosensory cortex (S1) layer 4 (L4) neurons. To which extent this high efficacy depends on structural synaptic features remains unclear. Using both serial transmission (TEM) and focused ion beam milling scanning electron microscopy (FIB/SEM), we 3D-reconstructed and quantitatively analyzed anterogradely labeled VPMdm axons in L4 of adult mouse S1. All VPMdm synapses are asymmetric. Virtually all are established by axonal boutons, 53% of which contact multiple (2-4) elements (overall synapse/bouton ratio = 1.6). Most boutons are large (mean 0.47 μm3), and contain 1-3 mitochondria. Vesicle pools and postsynaptic density (PSD) surface areas are large compared to others in rodent cortex. Most PSDs are complex. Most synapses (83%) are established on dendritic spine heads. Furthermore, 15% of the postsynaptic spines receive a second, symmetric synapse. In addition, 13% of the spine heads have a large protrusion inserted into a membrane pouch of the VPMdm bouton. The unusual combination of structural features in VPMdm synapses is likely to contribute significantly to the high efficacy, strength, and plasticity of these thalamocortical synapses.

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